The invention relates to a phase-locked loop frequency demodulator and frequency synthesizer for a frequency-modulated wave receiver of the type comprising two phase-locked loops or "PLL".
The use of phase-locked loops as frequency synthesizers in tuners (channel selectors) of radio frequency receivers is well-known and described, for example, in an article by BREEZE on pages 24 to 32 of the the U.S. periodical "IEEE TRANSACTIONS ON CONSUMER ELECTRONICS", vol. CE-24 No. 1 of February 1978, or by HILLIKER on pages 62 to 68 of Vol. CE-22, No. 1 of the same periodical, dated February 1976, or in the FR-A Nos. 2,236,318, 2,327,670 and 2,257,105 patent publications. Furthermore, their use as frequency-modulated wave demodulators is also well-known and described, for example, in articles by NIEPOLD on pages 245 to 253 of the German review "ARCHIVE DER ELEKTRONIK UND UBERTRAGUNGSTECHNIK , vol. 31, section 6, of June 1977, or by DOSWALD and WEHRLI on pages 96 and 106 of the Swiss review "TECHNISCHE MITTEILUNGEN PTT", vol. 52, No. 3, of March 1974, and by BREMENSON and DEHAENE on pages 479 to 509 of the French review "REVUE TECHNIQUE THOMSON-CSF', Vol. 10, No. 3, of September 1978, as well as in the corresponding FR-A No. 1,222,053 and U.S. Pat. No. 3,069,625 patent publications, as well as in the FR-A No. 1,443,419 or FR-A No. 2,180,001 patent applications.
The two above-mentioned uses for the phase-locked loop have also been described in articles by ZAKHEIM on pages 43 to 50 of the French review "TOUTE L'ELECTRONIQUE" No. 419 of March 1977 or by MURTHI on pages 59 to 64 of the U.S. review "EDN", vol. 22, No. 16, dated Sept. 5, 1977.
According to the prior art, frequency-modulated wave receivers may use two independent, complete phase-locked loops, one for frequency synthesis to tune the local oscillator of a heterodyne converter and the other for frequency demodulation, as illustrated in FIG. 1 of the accompanying drawings, which will be described further on. Such an arrangement has the disadvantage, that when the bandpass filtering elements ahead of the phase-locked loop demodulator input are insufficiently selective and/or when another received signal in a neighboring frequency channel is sufficiently strong, to allow the demodulator loop to lock onto the frequency of this other signal which is different from that chosen by means of the programmable divider of the frequency synthesizer, whose dividing factor is displayed as the number of the channel received. This latching onto a frequency different from the desired one is also possible when the voltage-controlled oscillator of the phase-locked loop demodulator is tunable by means of a voltage from a potentiometer or from a digitally controlled tuning voltage synthesizer, the tuning voltage being used to reverse bias the variable capacitor diode in the voltage controlled oscillator.
The combined arrangement of the tuning device and of the demodulator of the present invention avoids this disadvantage while providing an economy of means.
An oscillator circuit with digital stabilization is known and described in the publication U.S. Pat. No. 3,212,023, in which a first phase-locked loop used for frequency synthesis and comprising a first narrow-band low-pass loop filter, determines the oscillation frequency of a voltage-controlled oscillator by comparing the frequency of a wave supplied by a switchable divider fed by this oscillator, to a constant-frequency reference wave. This circuit further comprises a second, so-called auxiliary, phase-locked loop in which the phase of the wave supplied by the voltage-controlled oscillator is compared to that of a higher harmonic of the reference wave and the signal resulting from this comparison is applied through a second high-frequency loop filter, added to that from the first filter of the synthesis loop, to the frequency control input of the oscillator. Thus, the spurious phase modulation or "jitter" of the wave supplied by the oscillator, due to the switchable divider of the synthesis loop, can be reduced or eliminated.